I can't really see widespread use of flywheels. They are good for high power, low total amount of energy situations, namely frequency regulation.

Bearings and motors/generators are very well understood, unlike lithium ion batteries, so I don't see them getting much cheaper as a flywheel industry expands.

Currently, the most expensive lithium ion battery is cheaper than the cheapest flywheel:

https://www.lazard.com/media/2391/lazards-levelized-cost-of-...